Tunable Fabry-Perot filters (TFPFs) are narrowband wavelength filters that offer the advantage of high optical transmission, low wavefront distortion, low insertion loss, and easy tunability. Consequently, TFPFs are widely applied in fiber optic communications applications, such as WDM, wavelength locking, tunable filter, and the like. TFPFs have been used extensively in WDM systems as tunable de-multiplexers or noise filters within pre-amplified receivers. They are also used for online monitoring of optical channels in DWDM systems.
In order to transmit multiple DWDM channels, the finesse of the TFPF should be about 1000, with a channel spacing of 100 GHz (0.8 nm) and a pass bandwidth of up to 10 GHz (0.08 nm). Current TFPF's based on micro-electromechanical systems (MEMS) do not satisfy the requirements for DWDM applications due to low finesse. The surface quality of the mirrors determines the maximum possible finesse. It is difficult to achieve mirror surfaces in MEMS devices that are as flat and diffraction free as optically polished surfaces, and so it is harder to obtain as high a finesse as is possible with micro-optical components. Another problem for MEMS arises due to the lack of absolute stability of the structure. As a result, it is difficult to maintain perfect parallelism between the mirrors, at a specifically desired mirror separation. It is easy to produce a mirror tilt on the order of 0.01°, resulting in a reduction in the finesse by as much as 10%. Another problem for MEMS is related to the mirror reflectivity. A dielectric mirror having a reflectivity of about 95% can be deposited on a MEMS device without causing residual stress: this results in a maximum finesse of only about 61. Increasing the reflectivity by adding further dielectric coatings results in residual stress that causes deformation of the MEMS mirror, which degrades the finesse for coatings with a reflectivity of >95%.
Another approach to providing an TFPF is a piezo-scanned TFPF, in which the mirrors are provided on two separate substrates and one of the substrates is moved in relation to the other by a piezoelectric transducer. The piezo-scanned TFPF suffers from the normal hysteresis and nonlinearity associated with piezoelectric transducer, however. In addition, these devices have a problem of piezo creep, in which the position of the piezo-scanned mirror changes with time. Thus, the operation of the piezo-scanned TFPF is unstable. Consequently, the piezo-scanned TFPF requires a very complicated control system to compensate for the hysteresis and the creep. Another limitation to the piezo-scanned TFPF is the lack of high expansion coefficient of piezoelectric transducers. Currently, piezo-electric elements are limited to scan ranges of about 10 μm or less, where the applied voltages and device sizes are reasonable.